Sulfur-resistant composite metal membranes

ABSTRACT

The invention provides thin, hydrogen-permeable, sulfur-resistant membranes formed from palladium or palladium-alloy coatings on porous, ceramic or metal supports. Also disclosed are methods of making these membranes via sequential electroless plating techniques.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/768,188 filed on Jun. 25, 2007, which issued as U.S. Pat. No.8,101,243 on Jan. 24, 2012, which is a continuation-in-part of U.S.patent application Ser. No. 11/381,488, filed May 3, 2006, which is adivisional application of U.S. patent application Ser. No. 10/249,387,filed Apr. 3, 2003, now abandoned, which claims the benefit of priorityunder 35 U.S.C. §119(e) to U.S. Provisional Patent Application No.60/369,674, filed Apr. 3, 2002. This application also claims the benefitof priority under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication No. 60/805,723 filed Jun. 23, 2006. Each of these relatedapplications are incorporated herein by reference.

GOVERNMENT INTEREST

This invention was made with Government support under grant numberDE-FG26-03NT41792 awarded by the U.S. Department of Energy (from theUniversity Coal Research Program, grant number DE-FG03-93ER14363 awardedby the U.S. Department of Energy Office of Science and grant numberW56HZV-06-C-0077 awarded by the U.S. Department of Defense. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to thin, hydrogen-permeable, sulfur-resistantmembranes formed from palladium or palladium-alloy coatings on porous,ceramic or metal supports, methods of making these membranes, anddevices that incorporate these membranes.

BACKGROUND OF THE INVENTION

Inexpensive sources of purified hydrogen are sought after for manyindustrial chemical production processes and in the production of energyin fuel cell power systems. Similarly, inexpensive methods of purifyinghydrogen could significantly expand the applicability of hydrocarbonreforming, reforming reactors and the water gas shift reaction.

Palladium and its alloys, as well as nickel, platinum and the metals inGroups III-V of the Periodic Table are all permeable to hydrogen.Hydrogen-permeable metal membranes made of palladium and its alloys arethe most widely studied due to their high hydrogen permeability, theirchemical compatibility with many hydrocarbon containing gas streams, andtheir theoretically infinite hydrogen selectivity. Hydrogen molecules(H₂) present in a steam of mixed gas molecules will dissociate intohydrogen atoms, which dissolve into the palladium and diffuse across apalladium metal barrier to recombine into hydrogen molecules anddissociate from the opposite surface of the palladium barrier as apurified hydrogen gas. Thus, a gas stream formed in different industrialprocesses that contains many different molecular components includinghydrogen can be directed to a palladium membrane to selectively recoverthe hydrogen present in the gas, thereby producing a purified hydrogengas stream without significant additional energy input.

Unfortunately, pure palladium membranes are themselves expensive whenused in such purification processes due to their rapid degeneration andlimited life. Atomic hydrogen is so soluble in palladium that it forms aseparate hydride phase (β), which has a much larger lattice constant,causing swelling, warping and cracking of the palladium membrane. Thisα⇄β phase transition takes place at the critical temperature of 295° C.,making it difficult to avoid premature breakdown during prolongedindustrial use. Additionally, sources of sulfur, present in manyindustrial process gasses, produce hydrogen sulfide when they contactpalladium membranes at high temperature. Hydrogen sulfide is a potentpoison of the hydrogen dissociation catalysts including palladium metalmembranes, and exposure to sulfur-bearing gasses rapidly lowers thepermeability of a palladium membrane to hydrogen requiring thereplacement of the relatively expensive membrane structure.

In an attempt to overcome these problems with pure palladium membranes,alloys of palladium have been tested that display a comparable hydrogenpermeability with superior physical strength and greater resistance tothermal degradation and sulfur poisoning. As early as 1963, McKinley(U.S. Pat. No. 3,350,845) formulated alloys of palladium with copper,silver and gold and showed that palladium-gold alloys containing about55 weight percent gold had improved resistance to poisoning bysulfur-containing gases, albeit with about a 10-fold decrease inhydrogen permeability. Alternatively, palladium-silver membranes andpalladium-copper membranes containing about 10 weight percent silver andabout 40 weight percent copper, respectively, showed an increasedpermeability to hydrogen but were equally or more sensitive to sulfurpoisoning compared to pure palladium membranes.

The palladium-gold membranes disclosed by Mckinley were relatively thickand prepared by conventional metallurgy techniques. Such membranes arestill prohibitively expensive for most industrial applications.Therefore, there has been a long-felt need for a fabrication methodcapable of inexpensively and efficiently producing palladium alloymembranes having high thermal stability, durability and resistance tosulfide poisoning.

Recent research efforts have focused on the development of compositemetal membranes consisting of a relatively thin Pd or Pd-alloy coatingson hydrogen permeable base metals, or on porous ceramic or stainlesssteel supports.

Many palladium alloys such as Pd₇₃Ag₂₇, Pd₉₅Au₅, and Pd₆₀Cu₄₀ possesshigher hydrogen permeability than pure palladium. In the 1960s, McKinleyand coworkers (U.S. Pat. No. 3,439,474 (1969)) reported that binaryalloys of Pd with Au and Cu had pure hydrogen permeabilities greaterthan Pd and PdAg, were unaffected by thermal cycling, and had someresistance to sulfur poisoning by hydrogen sulfide. The inhibition orreduction of the pure hydrogen flux due to exposure to 4 ppm hydrogensulfide through the 40 mass percent Au alloy was the least compared topure Pd, PdAg and PdCu alloys.

The sulfur resistance of PdCu foil membranes was investigated byresearchers at the DOE NETL laboratory (B. D. Morreale, B. D, et al., J.Membr. Sci., 241:219 (2004)). They reported the best sulfur resistancewith a 20% Cu in Pd binary alloy having an FCC crystal structure. Butthis Pd₈₀Cu₂₀ alloy has only 20% of the hydrogen permeability of pure Pdand about 2 times less than 40% Au.

Thus, there is still a need for sulfur resistant, composite metalmembranes and improved methods of designing and making these membranes.

SUMMARY OF THE INVENTION

The present invention provides methods of fabricating a sulfur-resistantcomposite metal membranes including seeding a substrate with palladiumcrystallites, decomposing any organic ligands present on the substrate,reducing the palladium crystallites on the substrate to the metallicform, depositing a film of palladium metal on the substrate and thendepositing a second, gold film on the palladium film. These two metalfilms are then annealed at a temperature between about 200° C. and about1200° C. to form a sulfur-resistant, composite PdAu alloy membrane. PdAualloy layers formed by this method preferably have a thickness betweenabout 2 μm and about 10 μm and the substrate is preferably a stainlesssteel support having a porous ZrO₂ coating that may optionally be sealedso as to render one or more portions of the substrate impermeable tohydrogen gas.

The substrate may be seeded by airbrushing a palladium salt solutiononto the surface of the substrate. Preferably, a palladium salt solutioncontaining palladium (II) acetate and chloroform is used for thistechnique.

The substrate may be fired to eliminate any organic ligands present. Thesubstrate may also be reduced by immersion in a reducing solution suchas a solution containing water, NH₃OH and hydrazine.

The first film of palladium metal is deposited on the substrate byelectroless plating. In a preferred embodiment, a palladium plating bathsolution is pumped over the surface of the substrate, preferably until apalladium layer having a thickness of between about 1 μm and about 7 μmis formed. The palladium plating bath may be a solution containingwater, NH₃OH, HCl, Palladium (II) chloride and hyradzine. Similarly, thedeposition of a second film of gold is preferably performed by pumping agold plating bath, such as a solution containing water, NaOH and gold(III) chloride, over the surface of the substrate, and this is continueduntil a gold layer having a thickness of between about 1 μm and about 7μm is formed on the palladium layer.

Further metal layers of palladium, gold or other metals, such as silver,may be formed on the second, gold layer. Alternatively, palladium andsilver layers may be co-deposited on the substrate, followed by a layerof gold. Alternatively, palladium silver and gold layers may besequentially deposited on the substrate by sequential electrolessplating.

The invention also provides a sulfur-resistant PdAu composite membranesmade by seeding a substrate with palladium crystallites, optionallydecomposing any organic ligands present on the substrate, reducing thepalladium crystallites to the metallic form, depositing a first film ofpalladium metal on the substrate, depositing a second film of gold onthe palladium film and, annealing of the metal films to form a compositePdAu alloy membrane.

In the deposition of the metal membranes on the substrate during thefabrication methods of the present invention, the depositing steps arepreferably conducted in the absence of both organic complexing agents,such as EDTA, and tin.

The invention also provides hydrogen-permeable and sulfurpoisoning-resistant composite membranes. These composite membranes arecomposed of a porous substrate having a PdAu alloy layer on at least onesurface. The PdAu alloy preferably has a mass percent Au between 5 and50 mass percent and wherein the PdAu alloy is non-homogenous.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the effect of gas mixtures onthe flux as well as CO and CO₂ composition in the permeate for a testedPdAu composite membrane (GTC-75). H₂S was added to the baseline watergas shift mixture at 1 ppmv and 5 ppmv. T=400° C. and total feedpressure was 60 psig.

FIG. 2 shows the effect of time on the permeate flow rate from PdAucomposite membrane GTC-75 at 400° C. The water gas shift mixturecontained 51% H₂, 26% CO₂, 21% H₂O, and 2% CO. Typical feed pressure was60 psig.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides metal alloy membranes having highhydrogen permeability and good resistance to sulphide poisoning. Theinvention also provides optimal methods to fabricate sulfur resistant,high flux composite membranes that can be applied to high temperaturehydrogen separations.

One embodiment of the invention provides composite membranes composed ofa palladium alloy film supported on a substrate that display higher H₂flux from a multicomponent gas mixture containing hydrogen sulfide andother sulphurous constituents than the H₂ flux for previously reportedpalladium alloy composite membranes. These membranes are formed onporous supports by electroless plating methods.

The composite membranes include a thin palladium alloy layer on a poroussubstrate. The porous substrate may be any porous, inorganic supportincluding oxide ceramics (e.g., alumina, titania and zirconia),non-oxide ceramics (e.g., SiC and SiN), sintered or porous metals (e.g.,stainless steel and nickel), sintered or porous metals with ceramicsurfaces, and porous vycor glass. The substrate may be either tubular orplanar or any geometry such that the surfaces that bear the Pd alloyfilm are adequately exposed.

A permeable diffusion barrier separating the substrate from the Pd alloyfilm is desirable. For example, a stainless steel support coated withporous ZrO₂ having a pore diameter of about 80 nm has been extensivelytested and shown to work well.

Substrates that are symmetric or asymmetric, have pores of differentsizes, or have a gradient of pore sizes, may be used. While symmetricsubstrates are typically less expensive, asymmetric substrates have alower resistance to flow and therefore minimize the pressure dropexperienced in the support. Thus, given a similar thickness of Pd alloydeposited on the substrate, greater fluxes can be achieved with anasymmetric support. A small pore size in the substrate is needed tominimize surface roughness and therefore the corresponding Pd filmthickness. Alternatively, at pore sizes of 5 nm in diameter or less, theadhesion of the film to the substrate is reduced. Good results have beenobtained with asymmetric substrates with a pore size gradient thatextends over a 20-80 nm diameter pore size range.

Certain surfaces of the substrate may be sealed as desired to prevent H₂from flowing through those regions of the membrane. Typically, a lowtemperature glaze (e.g., potter's glaze) is utilized as the sealantbecause many substrates are subject to damage if exposed to hightemperatures.

The hydrogen flux through these Pd alloy membranes is inverselyproportional to the thickness of the membrane and therefore, thinner Pdalloy membranes are more desirable for their increased rate of hydrogenflux and lower cost for the palladium and alloy metal components. When asupport with small pores is used, a thinner film may be used and stillproduce a leak-free membrane, because it is easier to cap small pores byplugging them with metal. Very thin, defect-free Pd alloy membraneshowever, are difficult to form and the inherent defects lead to lossesin selective permeability of the membrane to hydrogen and more rapidphysical deterioration of the membrane, requiring more frequent membranereplacement. Thin metal films are sufficient to prevent leaking at lowtemperatures, but when heated, the metal crystallites may rearrange,opening slightly covered pores in the ceramic support, leading to theformation of pores or outright rupture of the palladium alloy membraneand loss of selective hydrogen permeability. Pd alloy membranes having athickness in the range of between about 1 nm and about 10 nm arepreferred as striking a balance between good hydrogen diffusion rates,relatively easy defect-free production and physical durability. Pd alloymembranes having a thickness in the range of between about 2 nm andabout 5 nm are the most preferred.

The weight percent content of the palladium alloy is formulated tomaximize hydrogen permeability while achieving resistance to sulfurpoisoning and physical durability. With PdCu alloys, the hydrogenpermeability increases through a maximum around 40 wt. % Cu (PdCu₄₀).This high percentage of Cu significantly reduces membrane cost relativeto pure Pd, and the PdCu₄₀ alloy exhibits increased resistance tohydrogen sulfide poisoning. Further, a PdCu₄₀ membrane can withstandrepeated temperature cycling with less distortion than pure Pd becauseat 40 wt. % Cu, the critical temperature for β-hydride phase formationis below room temperature. Similarly, PdAg₂₃ has a higher hydrogenpermeability than pure palladium membranes and an improved resistance tosulfide poisoning. Surprisingly, and in contrast to the PdAu membranesdisclosed by Mckinley (U.S. Pat. No. 3,350,845), PdAu membranes having alower weight percent Au in the range of Pd₉₅Au₅ to Pd₅₅Au₄₅ have ahigher hydrogen flux than pure palladium and a much greater resistanceto sulfide poisoning than comparable PdCu or PdAg membranes. Forexample, a 5 micrometer Pd₈₅Au₁₅ membrane shows only a 38% drop inhydrogen flux in the presence of 5 ppm H₂S compared to a 71% drop for acomparable Pd₉₄Cu₆ and the Pd₈₅Au₁₅ membrane had a higher hydrogen fluxin the presence of H₂S than the Pd₉₄Cu₆ membrane in the absence of H₂S.A Pd₈₀Au₂₀ membrane having a thickness of about 2.5 micrometers andformed on a stainless steel substrate with a ZrO₂ coating has a hydrogenpermeability approaching that of pure palladium and a hydrogen/nitrogenseparation factor of greater than the ideal value of 400 when tested at400° C. Similarly, a Pd₉₅Au₅ membrane having a thickness of about 3micrometers and formed on a stainless steel substrate with a ZrO₂coating had a hydrogen permeability approaching that of pure palladiumand a hydrogen/nitrogen separation factor of greater than 10,000 whentested at 400° C. A Pd₉₀Au₁₀ membrane formed on a stainless steelsubstrate with a ZrO₂ coating exposed to a water gas shift gas mixture(51% H₂, 26% CO₂, 21% H₂O, 2% CO) at 400° C. operated at a 76% H₂recovery which is nearly the same as the hydrogen flux seen withexposure to a pure gas flux having a similar H₂ partial pressure,indicating a hydrogen selectivity with only minor reductions in thepresence of a mixed molecular gas feed stream.

This hydrogen selectivity and substantial resistance to sulfidepoisoning is believed to result from the fabrication methods of thepresent invention that produce a PdAu alloy which is not perfectlyhomogenous on the surface of the substrate. The fabrication methodsdescribed below produce a partial PdAu alloy in which Au selectivelysegregates to the surface of the PdAu film, effectively forming pools ofAu on the free surface of the PdAu film. For instance, the presentinventors used Auger electron spectroscopy to show that a Pd₇₀Au₃₀ alloyat 250° C. actually contained 75 at % Au on its (111) surfaces and 80 at% Au on its (110) surfaces. Equivalent-Medium Approximation (EMA) wasused to accomplish this for a class of Pd—X alloys and the modelpredictions were found to compare favorably with ion scatteringexperiments. The segregation of Au to the Pd (111) surface is strongerthan that of Cu to the same surface. For instance, a 20 at % Auconcentration in the bulk is predicted to give 58 at % Au on thesurface, whereas a 20 at % Cu concentration in the bulk is predicted togive 20 at % Cu on the surface as well—i.e. there is no surfacesegregation of Cu at these concentrations.

Both Pd and Au form FCC structures throughout the temperature range of−73° C. to 900° C. Mixtures of Au and Pd are still FCC but show atendency to form ordered arrangements of atoms as the temperature islowered. Within a single FCC crystal containing both Au and Pd, theorder can be long range (LRO) or short range (SRO). The LRO is foundnear Au3Pd, AuPd, and AuPd3 stoichiometries. As the stoichiometry movesfrom one of these three regions, the LRO starts to deteriorate. Atstoichiometries far from these points, the structure is FCC with acompletely random arrangement of Pd and Au atoms. At intermediatestochiometries, though, there is some “clumping” of the Au and Pd atoms,and this is the SRO order mentioned above. Both experimental and abinitio investigations by the present inventors have shown that such SROexists at stochiometries and temperatures of interest in the commercialoperation of the composite metal membranes of the present invention.These data indicate that 20% Au within the bulk results in 80% Au on(111) surfaces. These experimental and computational results indicatethat monolayers with roughly this fraction of Au atoms will form SROstructures that are desirable from a functionality standpoint.

Without intending to be bound by any one theory, the present inventorsbelieve that this partial segregation phenomenon accounts for thereduced hydrogen permeability of the PdAu composite membranes tested andalso to the substantially enhanced resistance to sulfide poisoning seenwith the PdAu membranes of the present invention. Sulfide poisoning ofmembrane surfaces has been found to correlate with the binding energy ofsulfur atoms to the atoms at the surface of the metal membranes. Studiesby the present inventors have revealed substantial overlap of sulfurp-states with the palladium surface d-band giving rise to a strongcovalent bond between the adsorbate and the surface. This is presumablywhy the sulfur is so pernicious; the sulfur is a very strong siteblocker. Comparing the binding energy between the sulfur atoms and thesurface atoms of Pd, Cu, Au, Pd—Cu alloy and Pd—Au alloy membranes,Pd—Au alloys have the lowest binding energy while Pd metal membraneshave the highest binding energy (ordered from of Pd>Cu>Pd—Cualloys>Au>Pd—Au alloys). Density Functional Theory (DFT) simulationdetermined that sulfur atoms settled in the FCC sites, as expected. Thebond length between a sulfur atom and its nearest Pd neighbor wasdetermined to be of 2.442 Å. Other studies have found a lower value for25% coverage of sulfur: 2.27 Å (DFT) and 2.23-2.38 Å (experimental).This trend makes sense since the higher coverage causes sulfur atoms torepel each other, reducing the binding energy and extending the bondlength. The test was repeated for sulfur binding to a pure Cu sample toshow a binding energy of 4.49 eV/atom. The DFT binding energy analysiswas then performed again with 100% Au coverage on the Pd (111) surface.While this would never be used as a hydrogen separation membrane, theanalysis was intended to serve as a basis for assessing how low thebinding energy of sulfur could go in this Pd—Au system. The associatedbinding energy was determined to be only 2.085 eV—less than half of thatassociated with the Pd—S surface. This indicates the potential of thePd—Au alloy to resist sulfide poisoning. The (111) surface was thenmodified to have a 50% Au coverage. As expected, the relaxed system ischaracterized by Au—S bond lengths (2.884 Å) that are greater than theirPd—S counterparts (2.437 Å). Significantly though, the average bindingenergy of sulfur to this alloyed surface is 2.573 eV—much closer to thatof the pure Au surface than that of the pure Pd. In other words, theclusters of Au on the Pd surface have caused a reduction in the sulfurbinding energy that is not in proportion to the amount of Au present.This result may be related to the promotional effect of Au in formingvinyl acetate (VA) in Pd—Au alloys.

Au nanoclusters exhibit strong catalytic behavior for a CO oxidation,and vinyl acetate synthesis, and this has prompted a flurry of researchactivity. This is germane to the Pd—Au membranes since ordered Au atomson the Pd surface may be thought of as nanoclusters of Au. Particularlyrelevant to the function of the membrane, though, is role of Au ininfluencing the rate of H₂ adsorption. Au clusters on Pd, Pd clusters onAu, and Pd overlayers on Au substrates also have potential applicationsfor systems in exhaust cleanup and fuel cells. These efforts form abasis for a consideration of how hydrogen adsorption is influenced bythe presence of Au in PdAu membranes. The difference is that Au clustersin the membranes are short-ranged ordered atoms within the Pd latticewith either smooth or restructured single-crystal surfaces. The studiesby the present inventors indicate that the presence of 10-20 weightpercent Au actually increases the permeability of hydrogen, and this maybe due to the nano-catalytic properties of Au on the membrane surface.

Nudged elastic band (NEB) transition state theory was implemented withinthe DFT analysis to estimate the reaction barrier and adsorption energyof H₂ on Pd—Au surfaces. Only two (111) layers were considered, and thebottom layer was held fixed during H adsorption. It was first verifiedthat adsorption to neighboring FCC sites for a pure Au membrane isendothermic (11.67 kcal/mol) while the same process on pure Pd membranehas a negative adsorption energy (21.92 kcal/mol). Significantly though,a 50% coverage of Au was found to be exothermically adsorbed H₂ (−11.27kcal/mol). This is consistent with experimental data, which indicatesthat H permeability is reduced, but not destroyed, by the presence ofhigh Au surface coverage. Roughening of the surface so as to allowout-of-plane clusters of Au atoms will only increase the catalyticeffect of the Au.

Molecular dynamics (MD) simulation tools were employed to study thediffusion of H atoms through Pd—Au membranes in order to betterunderstand the role of the Au atoms. Within the MD paradigm, atoms areviewed as point masses with forces stipulated among atoms of differentspecies. These forces can include bond order terms and, in that case,can even allow for chemical reactions. MD was employed to study a singleH atom moving through a block of six unit cells of Au, Pd, and AuPd3crystals. The simulation times tend to be extremely short—on the orderof 1 nanosecond—and so the lattices were dilated in order to increasethe diffusivity. Qualitative conclusions could then be made about howthe diffusivity of H is influenced by stoichiometry. It was found that Htends to get trapped in small clusters of Au atoms and that this reducesthe overall diffusivity of the metal. This conclusion is consistent withexperimental evidence. Typically, a single hydrogen atom is allowed todiffuse through a Pd—Au crystal. It was observed that the H atom spentnearly all of its time in orbital trajectories between pairs of Auatoms. Long orbital periods were followed by a rapid excursion to a new,equivalent orbital site.

The coverage dependent desorption of H from the downstream side of Pd—Aumembranes was studied using the same approach as for adsorption. In thiscase, it is the energy barrier that is of interest. For example, theassociated desorption of H was studied using a two-layer structure ofPd(111) with a surface composition of Pd_(0.5)Au_(0.5). Two cases wereconsidered: 50% and 100% initial coverage of H on the preferred FFCsites. The study indicates that both the desorption and adsorptionbarriers increase significantly with coverage, with the net effect beinga slower rate of H desorption at high coverages.

Thus, one embodiment of the invention provides a method of making thesecomposite Pd—Au alloy membranes. Both planar and tubular Pd/Au membranescan be fabricated using the improved, sequential, electroless platingprocesses described here. In a preferred embodiment, tubular Pd/Aumembranes are formed on stainless steel supports. PdAu alloy membranesare deposited onto ZrO₂/stainless steel substrates by a sequentialelectroless plating process. Although other deposition methods may beused, electroless plating offers advantages over other depositiontechniques because it can deposit uniform films on complex shapes andlarge substrate areas with sufficient hardness, using simple equipment.PdAu alloy membranes are fabricated by sequential plating of first Pdand then Au. The target thickness for the PdAu alloy films is between 2μm and 5 μm to maximize the hydrogen permeance.

This method advantageously eliminates tin (Sn) and carbon impurities inthe Pd films, which can cause structural instability, particularly athigh temperature, and reduced H₂/N₂ separation ratio (pure gaspermeability ratio). These methods are particularly suited for alloycompositions with 5-50 mass % Au, which the present inventors have shownto have the highest permeability and best resistance to sulfur.Importantly, no organic complexing agent, such as EDTA, is used in thePd plating solution to minimize contamination by carbon. When pure gasesare used, they are preferably nominally 99.999% pure (UHP grade). Theprocess includes sequentially depositing Pd and then the alloying metal,Au or Cu, using electroless plating, followed by a high temperatureanneal to allow intermetallic diffusion of Pd and the alloying metal.

Initially, a substrate is provided. If needed, the provided substrate issubjected to a pre-processing step in which the substrate is subjectedto one or more operations that are needed to place the substrate incondition for plating related operations and/or one or more operationsthat are more readily accomplished prior to plating related operations.If the substrate that is provided is not clean or becomes dirty before aplating operation, the substrate must be cleaned to remove any salts orother materials that could interfere with the subsequent platingprocesses. Typically, cleaning is carried out with isopropanol anddeionized water but other cleaning procedures that remove theundesirable material or materials are also feasible. Further, if thesubstrate that is provided does not have the appropriate dimensions,appropriate sizing operations are undertaken. Typically, this involvescutting the substrate but other form- or shape-altering methods are alsofeasible. It is also feasible to perform sizing operations at differentpoints in the composite membrane production process.

In these substrate preparation processes, the surfaces of the substrateare sealed in places where it is undesirable to have H₂ flow in thefinished membrane. Typically, a low temperature glaze (e.g., potter'sglaze) is utilized because many substrates are subject to damage ifexposed to high temperatures. For example, if the U.S. Filter T1-70 5 nmfilter, an asymmetric ceramic filter, is exposed to temperatures above600° C., the thin top layer of the filter is subject to damage.Regardless of the sealant utilized, the sealant is either painted ontothe surface to be sealed or the surface is dipped in sealant. Othermethods, such as spraying, are also feasible. In a preferred embodiment,the ends of a tubular substrate are each dipped into the sealant. Withthe ends sealed and assuming that the Pd alloy film is going to beapplied to the outer wall of the substrate, H₂ and other materials thatare in a stream that is passing through the tubular substrate areconstrained to traveling through the inner wall of the substrate to theouter wall of the substrate, and then through the Pd alloy film on theouter wall of the substrate to the appropriate collection on the outsideof the tubular composite membrane. Because of the sealant, the H₂ andother materials are prevented from exiting the substrate via the endwalls. It is also feasible to perform sealing operations at a differentpoint in the production process. For instance, in the case of a metalsubstrate, sealing by brazing, silver soldering or welding are feasibleat any point in the production process.

After pre-processing of the substrate, the surface of the substratewhere the Pd alloy film is to be deposited is “seeded” with Pdcrystallites to catalyze the rate of film growth (i.e. heterogeneousnucleation as opposed to homogenous nucleation) on the substrate andachieve good adhesion of the palladium “seeds” to the substrate byemploying palladium seeds that are smaller than the grains on thesurface of the substrate that is being plated. This process can beperformed using a variety of methods including impregnation with anorganic solution of Pd acetate. In a preferred embodiment, the membranesupports are seeded with palladium nanocrystallites by airbrushing asolution of palladium salt onto the surface of the chosen support. Inthe case of a metal substrate, the seeding of the surface with Pdcrystallites may be accomplished using an organic Pd solution in whichthe solvent is a light, polar, organic solvent, such as tetrahydrofuran(THF), ethyl acetate, acetonitrile, diethyl ether, methyl ethyl keton(MEK), or acetone. Preferably, the solvent is chloroform. Thecomposition of a preferred solution or “activation bath” for seedingsubstrates with palladium nanocrystals is shown in Table 1.

TABLE 1 Component Quantity Palladium (II) Acetate, 99% pure  3.3 gramsChloroform, HPLC grade 100 mL

The membrane is then fired in air in order to decompose any organicligand present. Air firing at about 350° C. for about 5 hours istypically sufficient to eliminate the acetate ligand present in theactivation bath of Table 1, but one of skill in the art will readilyrecognize that variations on this time and temperature can be made whilestill accomplishing the decomposition of organic ligands. The activationprocess may be performed more than once and is preferably performedtwice.

Prior to any plating process, the activated membrane is reduced in orderto convert the palladium crystallites to the metallic form. This may beaccomplished by immersion in a dilute hydrazine solution for 20 minutesat 50° C., but it will be readily recognized that variations on thisreduction process are possible while still accomplishing the reductionof the crystallites to the metal form. A preferred hydrazine solutionused in reducing the palladium crystallites is provided in Table 2.

TABLE 2 Component Quantity Deionized H₂O 650 mL/L 28-30 wt % NH₃OH 340mL/L Hydrazine (3M)  10 mL/L

After the portion of the surface that is to bear the Pd alloy film hasbeen seeded, a Pd film is deposited on the seeded surface. Electrolessplating systems, such as non-flowing plating, batch plating and vacuumpump plating, may be used to deposit the Pd film. In one embodiment, thePd film is deposited using a flow system with an osmotic pressuregradient. In another embodiment, a plating bath is prepared containingpalladium chloride and the bath is pumped over the surface of theactivated and reduced membrane. A preferred palladium plating bathcomposition is provided in Table 3.

TABLE 3 Component Quantity Deionized H₂O 602 mL/L 28-30 wt % NH₃OH 392mL/L 37 wt % HCl  6 mL/L Palladium (II) Chloride, 99% pure  5.5 g/L

3 M hydrazine is added to the plating bath shown in Table 3 immediatelyprior to plating, with a volume ratio of 100 parts plating bath to 1part hydrazine. The ratio of the volume of Pd plating solution to thearea to be plated ranges from about 3 cm³/cm² to about 5 cm³/cm². Thebath is then heated to about 50° C. and pumped over the surface of theactivated and reduced membrane. The plating cycle continues for a timesufficient to produce the desired thickness of the palladium film. Thedeposited film is typically in the range of about 1 μm to about 7 μmthick. A typical plating cycle lasts about 20 minutes to produce a Pdlayer having a thickness of about 1 μm.

A metal to be alloyed with the palladium layer is then plated on thepalladium-plated substrate. This metal(s) may also be applied byelectroless plating. The electroless plating may be conducted in thesame manner as the plating of the palladium plating operation. In thecase of gold plating, a preferred gold plating bath is provided in Table4.

TABLE 4 Component Quantity Gold (III) Chloride, 99% pure  1 g/L 50 wt %Sodium Hydroxide  20 mL/L Deionized H₂O 980 mL/L

In one embodiment, gold plating is performed by flowing the gold platingbath on the film (activated) side of the membrane, while the reducingbath from Table 2 is simultaneously flowed on the support side of themembrane. This is done at 50° C. and plating continues until a uniformfilm of gold of a desired thickness is formed on the palladium membranesurface.

If another alloy metal is to be added over the first alloy metal layer,it is added following the application of the first alloying metal. Forexample, if silver plating is to be conducted in place of the gold layerdescribed above (i.e. if a silver layer is to be applied to thepalladium layer) or if a silver layer is to be placed over the goldlayer described above (i.e. if a silver layer is to be applied to thegold layer that has been established on the palladium layer, with theintent of eventually forming a ternary Pd—Au—Ag alloy membrane), thatsilver layer is also added using any of the electroless plating methodsnoted above. A preferred silver plating bath is described in Table 5.

TABLE 5 Component Quantity Silver Nitrate, 99.9% pure 0.31 g/L 28-30 wt% NH₃OH  780 mL/L Deionized H₂O  220 mL/L

In one embodiment, the silver plating bath of Table 5 is used and 0.3 Mhydrazine is added to the plating bath immediately prior to plating, ina volume ratio of 50 parts plating bath to 1 part hydrazine. The bath isthen heated to 40° C. and flowed over the surface of the activated andreduced membrane. This plating cycle is conducted for a time sufficientto produce a sufficiently-thick silver layer and typically lasts about30 minutes.

In one alternative embodiment, a Pd—Ag alloy membrane may also beproduced in one step using the palladium-silver plating bath provided inTable 6.

TABLE 6 Component Quantity Silver Nitrate, 99.9% pure 0.26 g/LTetraaminepalladium 1.37 g/L chloride, 99.9% pure 28-30 wt % NH₃OH  372mL/L Deionized H₂O  628 mL/L Hydrazine (3M) 3.33 mL/L

In this embodiment an appropriately activated substrate is used and theplating temperature is typically about 40° C.

One way to achieve Pd and other metal layers of approximately equalthickness is by performing the metal plating operations undersubstantially the same thermodynamic conditions and for appropriateperiods of time. It is also possible to perform the plating operationsunder different thermodynamic conditions and/or over different periodsof time and achieve layers of substantially equal thickness. As notedabove, the maximum hydrogen flux for Pd alloys is achieved withdifferent weight percentages of different metal constituents.Consequently, the conditions under which weight percentages are achievedthat are at, or near, the weight percentages for maximum hydrogen fluxare typically different than those for the PdAu alloy.

After the Pd and other metal layers have been plated onto the substrate,the structure is subjected to an annealing operation sufficient toachieve at least some intermetallic diffusion of the metal(s) layer intothe Pd layer. In one embodiment with a gold and palladium layer,annealing is accomplished by slowly heating the structure to atemperature between about 350° C. and about 600° C., depending upon thethickness of the gold and palladium layers, in a hydrogen atmosphere.The annealing step permits intermetallic diffusion of the Au layer intothe Pd layer forming the alloy. Annealing progress may be observed bymeasurement of the H₂ flux. When the flux reaches a steady-state value,the annealing process is complete. For thin, 1 μm films, this processwill require about 24 hours at the lowest temperature. At this point,the PdAu or PdAuAg composite membranes are complete and may be used inhydrogen separation applications that involve streams of hydrocarbonsthat include sulfur or sulfur compounds.

An optional further step may increase the hydrogen flux of the membrane.Namely, the composite membrane may be subjected to air oxidation andreduction to activate the metal surface. This step is believed toroughen the surface of the film, thereby increasing the surface area ofthe film. The increased surface area is believed to provide a greaterhydrogen flux. The increased surface is believed to be responsible forthe greater hydrogen flux. In one embodiment, a short duration (5 to 30minutes) air oxidation at temperatures above 350° C. followed byexposure to H₂ and subsequent reduction. Rather than air reduction andoxidation, the composite membrane can be subjected to O₂, O₃, acids,steam, SO₂, or a combination of H₂S/steam to disturb the surface of thepalladium alloy film.

Additional objects, advantages, and novel features of this inventionwill become apparent to those skilled in the art upon examination of thefollowing examples thereof, which are not intended to be limiting.

EXAMPLES

In this investigation, the H₂ flux from a water gas shift (WGS) mixturecontaining H₂S was evaluated for PdAu alloys and compared with FCC PdCualloy composite membranes previously described in the literature (F.Roa, J. D. Way, S. DeVoss, G. Alptekin. Proceedings of ICIM 8,Cincinnati, Ohio, 2004).

Example 1 Membrane Fabrication

Composite Pd—Au alloy membranes were fabricated by sequential depositionfrom palladium and gold electroless plating baths onto symmetric 0.2 μmcut-off α-alumina tubes (GTC-200) fabricated by Golden TechnologiesCompany. No EDTA was used in the Pd plating solution to minimizecontamination by carbon. When pure gases were used they were nominally99.999% pure (UHP grade).

Prior to palladium plating, the surface of the membrane support wasfirst seeded with Pd particles by impregnating the ceramic support withan organic Pd salt solution, followed by reduction in aqueous hydrazinesolution. Pd and Au electroless plating baths were then used in sequenceto deposit films ranging from 5 to 7 microns in thickness.

Example 2 High Temperature Permeation Tests

The membrane to be tested was loaded into a stainless steel module,which in turn was mounted in a tube furnace. To avoid embrittlement, themembranes were heated under helium and no H₂ was introduced until themembrane reached 350° C. Annealing the two metals was achieved duringthe initial single gas permeability tests. Transmembrane pressuredifferentials varied from a few bars to as high as 7.7 bars (100 psig),while typical operating temperatures varied from 350° C. to 450° C.Permeate pressure was local atmospheric pressure in Golden, Colo. (˜0.8bar absolute or bara=12 psia).

Example 3 Membrane Characterization

Scanning electron microscopy (SEM) was utilized to determine filmthickness and surface structure; X-ray diffraction (XRD) and energydispersive X-ray spectroscopy (EDAX) were used to study crystalstructure and determine Pd—Au alloy composition.

Given the data reported by McKinley (U.S. Pat. No. 3,350,845) that PdAualloys were superior to PdCu in terms of resistance to poisoning by H₂S,several PdAu alloy membranes on porous ceramic supports were made byelectroless plating methods. An example is membrane GTC-31 which has analloy composition of approximately 19 mass % Au in Pd as determined byEDAX. The estimated thickness of this membrane, from mass gain, is 7 μm.The ideal H₂/N₂ selectivity using stainless steel compression fittingsand graphite ferrules for this membrane ranged from 1020 at 100 psi to1320 at 10 psi.

An additional PdAu/alumina composite membrane (GTC-75) was prepared thatwas similar to GTC-31. The estimated thickness of GTC-75 is 5 micronsfrom mass gain. From EDAX, this membrane had an average Au content of 15mass %.

Permeation studies with a simulated equilibrium mixture from the watergas shift containing 51% H₂, 26% CO₂, 21% H₂O, and 2% CO were performedwith PdAu membrane GTC-75 at a total pressure of 72 psia at 400° C. Themembrane area for GTC-75 was 10 cm². When mixture experiments with theWGS feed containing 2% CO were repeated with this membrane, essentiallyno inhibition or reduction of the hydrogen flux was observed. Going froma pure H₂ feed to the WGS mixture where the hydrogen partial pressure inthe feed gas was held constant, there was no change in the hydrogenflux. For example, the pure H₂ flux measured for GTC-75 when the feedpressure was 25 psig was 0.22 mol/m²s.

The total permeate flux as well as the CO and CO₂ compositions in thepermeate for membrane GTC-75 for a variety of feed gas mixtures wasdetermined and is shown graphically in FIG. 1. The first feed gasmixture tested was the water gas shift mixture containing 51% hydrogenat 72 psia pressure. The permeate flux was 0.2 mol/m²s at a hydrogenpartial pressure difference of approximately 25 psi. This wasessentially the same flux measured for pure hydrogen at a similartemperature and H₂ partial pressure. The CO composition in the permeatewas 180 ppmv and the average CO₂ composition was approximately 0.5%. Thepermeate hydrogen purity was 99.5%.

When 1 ppmv H₂S was added to the water gas shift mixture, there was a23% reduction in the permeate flow or hydrogen flux. The inhibition, orreduction in hydrogen flux due to H₂S, increased to 38% when the H₂Sfeed composition was increased to 5 ppmv. The 23% flux reduction for thePdAu membrane with 1 ppm H₂S is considerably less than the 38% drop inH₂ flux we previously observed for a Pd₉₃Cu₇ alloy composite membranetested at 350° C. with the same feed composition. Also, the H₂ flux forGTC-75 without sulfur was almost twice that of the Pd₉₃Cu₇ membrane eventhough the H₂ partial pressure difference was about 20% of the priorcase.

A plot showing the stability of the hydrogen flux of membrane GTC-75 isshown in FIG. 2. Noise in the data shown in FIGS. 1 and 2 were caused bydifficulties in controlling pressure when steam was present. Thestability of the hydrogen flux of membrane GTC-75 was tested for 475hours (or 20 days) continously and ultimately, about a 10% reduction influx (from 0.2 to about 0.18 mol/m²s) was observed. When the membranewas removed from the test module after 20 days of testing, the surfaceappeared unchanged from the sulfur exposure, and was palladium coloredand shiny. An SEM image of the surface of the membrane after testingshowed no pores formed on the membrane surface.

Two summarize these results, two PdAu composite membranes supported on0.2 micron porous alumina tubes were fabricated by sequentialelectroless plating and annealing and tested with pure H₂, a water gasshift mixture, and the WGS mixture containing 1 ppmv and 5 ppmv H₂S. Noflux reduction was observed for the WGS mixture compared to a pure H₂feed gas at the same 25 psig partial pressure. A typical pure H₂ fluxwas 0.8 mol/m²s for a 100 psig H₂ feed gas pressure at about 400° C. A23% and 38% decrease in the flux for the gas mixtures containing 1 ppmvand 5 ppmv sulfur, respectively, were observed, but in both cases themembrane achieved a stable flux for over 100 hours of testing. A 99.5%pure H₂ stream was produced from the WGS mixture.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and the skill or knowledge of the relevant art, arewithin the scope of the present invention. The embodiment describedhereinabove is further intended to explain the best mode known forpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with variousmodifications required by the particular applications or uses of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

1. A sulfur-resistant palladium alloy composite membrane comprising: apalladium layer that makes contact with a surface of a porous substrate;a second layer comprising a second layer material that makes contactwith the palladium layer, wherein the second layer material is selectedfrom the group consisting of copper, silver and gold; and a third layercomprising a third layer material that makes contact with the secondlayer, wherein the third layer material is selected from the groupconsisting of copper, gold and silver and wherein the second layermaterial and the third layer material are different.
 2. The membrane ofclaim 1, wherein the second layer material is copper and the third layermaterial is selected from the group consisting of gold and silver. 3.The membrane of claim 1, wherein the second layer material is gold andthe third layer material is selected from the group consisting of silverand copper.
 4. The membrane of claim 1, wherein the second layermaterial is silver and the third layer material is selected from thegroup consisting of gold and copper.
 5. The membrane of claim 1, whereinthe porous substrate is selected from the group consisting of an oxideceramics, alumina, titania, zirconia, non-oxide ceramics, SiC, SiN,sintered metal, porous metal, stainless steel, nickel, sintered metalswith ceramic surfaces, porous metals with ceramic surfaces, and porousvycor glass.
 6. The membrane of claim 1, wherein the porous substrate istubular.
 7. The membrane of claim 1, wherein the porous substrate isplanar.
 8. The membrane of claim 1, wherein the porous substrate is astainless steel support.
 9. The membrane of claim 8 further comprising aporous ZrO₂ coating on the stainless steel support.
 10. The membrane ofclaim 1, further comprising a sealant.
 11. The membrane of claim 1,wherein a thickness of the palladium first layer is between about 1 μmand about 7 μm.
 12. The membrane of claim 1, wherein the second materiallayer is gold and a thickness of the second layer is between about 1 μmand about 7 μm.
 13. The membrane of claim 1, wherein a pore size of theporous substrate is between 20-80 nm.
 14. A sulfur-resistantpalladium-gold alloy composite membrane comprising: a palladium firstlayer that makes contact with a surface of a porous substrate, whereinthe palladium-gold alloy, wherein a thickness of the palladium firstlayer is between about 1 μm and about 7 μm; and a gold second layer thatmakes contact with the palladium first layer, wherein a thickness of thegold second layer is between about 1 μm and about 7 μm.
 15. The membraneof claim 14, wherein a thickness of the palladium-gold alloy is betweenabout 2 μm and about 10 μm.
 16. The membrane of claim 14, wherein athickness of the palladium-gold alloy is between about 2 μm and about 5μm.
 17. The membrane of claim 14, further comprising: a third layer thatmakes contact with the second layer, wherein the third layer comprisinga third layer material selected from the group consisting of copper andsilver.
 18. The membrane of claim 14, wherein mass percent of the goldin the palladium-gold alloy is between 5 and 50 mass percent.
 19. Themembrane of claim 14, wherein the porous substrate is tubular.
 20. Themembrane of claim 14, wherein the porous substrate is planar.
 21. Asulfur-resistant a palladium alloy composite membrane comprising: aporous substrate; and a palladium alloy in contact with at least onesurface of the porous substrate, comprising palladium; a second materialselected from the group consisting of copper, silver and gold; and athird material, selected from the group consisting of copper, gold andsilver; wherein the second material and the third material are differentfrom each other, and wherein a thickness of the palladium alloy isbetween about 2 μm and about 10 μm.